Elastomers vulcanisation

Sircar [138] has reviewed the analysis of elastomer vulcanisate composition by TG/DTG techniques. The classical ASTM method, D297-93 [139], is too lengthy to be of much practical use on a routine basis, often requires preliminary identification of the polymer and is costly. TG has gained itself wide acceptance as a method for quantitative compositional analysis of vulcanisates ASTM El 131 [140], is basically designed for the analysis of rubber compounds [141]. Thermogravimetric analysis can be used to determine ... 

The incorporation of carbon black into elastomeric systems is a process of significant commercial importance. However, the additional stiffness of the sample imparted by the reinforcement effect of fillers is not favourable in terms of the experimental conditions for high-resolution NMR spectroscopy. Electric conductivity of the carbon black may also interfere to some extent. Under these circumstances, filled formulations are not widely used for the study of elastomer vulcanisations where high resolution and signal-to-noise ratios are required to detect small amounts of vulcanisation products. 

A major challenge in TG analysis of elastomer vulcanisates is to accurately separate oil/plasticiser and elastomer regions, which often overlap. Most of these materials have volatilisation ranges rather than discrete volatilisation points because they are... 

Initially, vulcanisation was accomplished by heating elemental sulfur at a concentration of 8 parts per hundred parts of rubber (phr) for 5 h at 140 °C. The addition of zinc oxide reduced the time to 3 h. Accelerators in concentrations as low as 0.5 phr have since reduced times to 1-3 min. As a result, elastomer vulcanisation by sulfur without accelerator is no longer of commercial significance. An exception is the use of about 30 or more phr of sulfur, with little or no accelerator, to produce moulded products of a hard rubber called ebonite. 

As with all thermoplastic elastomers, the copolyesterethers can be processed as thermoplastics. They are linear polymers and contain no chemical cross-links, thus the vulcanisation step needed for thermosetting elastomers is eliminated and scrap elastomer can be re-used in the same process as virgin material (176—180). 

Uretha.nes. Urethane elastomers are prepared by the reaction of an isocyanate molecule with a high molecular weight ester or ether molecule. The result is either an elastomeric mbber form or a Hquid prepolymer that can be vulcanised with an amine or a hydroxyl molecule (see Urethane POLYAffiRS). 

Peroxides. Peroxides are probably the most common materials used after sulfur because of their abiUty to cross-link a variety of diene- and non diene-containing elastomers, and their abiUty to produce thermally stable carbon—carbon cross-links. Carbon—carbon bonds are inherently stronger than the carbon—sulfur bonds developed with sulfur vulcanisation (21). 

Silicone Heat-Cured Rubber. Sihcone elastomers are made by vulcanising high molecular weight (>5 x 10 mol wt) linear polydimethylsiloxane polymer, often called gum. Fillers are used in these formulations to increase strength through reinforcement. Extending fillers and various additives, eg, antioxidants, adhesion promoters, and pigments, can be used to obtain certain properties (59,357,364). 

The earliest study describing vulcanised polymers of esters of acryUc acid was carried out in Germany by Rohm (2) before World War I. The first commercial acryUc elastomers were produced in the United States in the 1940s (3—5). They were homopolymers and copolymers of ethyl acrylate and other alkyl acrylates, with a preference for poly(ethyl acrylate) [9003-32-17, due to its superior balance of properties. The main drawback of these products was the vulcanisation. The fully saturated chemical stmcture of the polymeric backbone in fact is inactive toward the classical accelerators and curing systems. As a consequence they requited the use of aggressive and not versatile compounds such as strong bases, eg, sodium metasiUcate pentahydrate. To overcome this limitation, monomers containing a reactive moiety were incorporated in the polymer backbone by copolymerisation with the usual alkyl acrylates. 

More specific recipes appear in Table 3. The ingredients are added to the elastomers on standard two-roU mills or in internal mixers. Finished compounds are readily extmded, calendered, or molded in standard equipment. Vulcanisation of extmdates is accompHshed in Hve steam autoclaves, Hquid salt baths, fluidized beds, and microwave equipment. 

Fig. 4. Phase arrangement in hard polymer/elastomer combinations in which the elastomer phase has been dynamically vulcanised.

Subsequently, much improved thermoplastic polyolefin rubbers were obtained by invoking a technique known as dynamic vulcanisation. This process has been defined (Coran, 1987) as the process of vulcanizing elastomer during its intimate melt-mixing with a non-vulcanizing thermoplastic polymer. Small elastomer droplets are vulcanized to give a particulate... 

Although the elastomer phase is essentially in particulate form, the tensile strength of the blend can be increased five-fold by increasing the cross-link density from zero to that conventionally used in vulcanisation processes, whilst tension set may be reduced by over two-thirds. Since the thermoplastic polyolefin phase may be completely extracted by boiling decalin or xylene, there is apparently no covalent chemical bonding of elastomer and thermoplastic phases. 

Vulcanisation can be effected by diamines, polyamines and lead compounds such as lead oxides and basic lead phosphite. The homopolymer vulcanisate is similar to butyl rubber in such characteristics as low air permeability, low resilience, excellent ozone resistance, good heat resistance and good weathering resistance. In addition the polyepichlorohydrins have good flame resistance. The copolymers have more resilience and lower brittle points but air impermeability and oil resistance are not so good. The inclusion of allyl glycidyl ether in the polymerisation recipe produces a sulphur-curable elastomer primarily of interest because of its better resistance to sour gas than conventional epichlorhydrin rubbers. 

Early examples of such branched polysulphides, e.g. Thiokol FA, are believed to possess hydroxyl end groups and are coupled by means of zinc compounds such as the oxide, hydroxide, borate and stearate by a mechanism which is not understood. Later elastomers, e.g. Thiokol ST, have been modified by a restricted reductive cleavage (see below) and this generates thiol (mercaptan) end groups. These may be vulcanised by oxidative coupling as illustrated below with lead peroxide ... 

Oil resistance demands polar (non-hydrocarbon) polymers, particularly in the hard phase. If the soft phase is non-polar but the haid phase polar, then swelling but not dissolution will occur (rather akin to that occurring with vulcanised natural rubber or SBR). If, however, the hard phase is not resistant to a particular solvent or oil, then the useful physical properties of a thermoplastic elastomer will be lost. As with all plastics and rubbers, the chemical resistant will depend on the chemical groups present, as discussed in Section 5.4. 

In general, the thermoplastic elastomers have yet to achieve the aim of replacing general purpose vulcanised rubbers. They have replaced rubbers in some specialised oil-resistant applications but their greatest growth has been in developing materials of consistency somewhat between conventional rubbers and hard thermoplastics. A number of uses have also been developed outside the field of conventional rubber and plastics technology. 

A manufacturer considering using a thermoplastic elastomer would probably first consider one of the thermoplastic polyolefin rubbers or TPOs, since these tend to have the lowest raw polymer price. These are mainly based on blends of polypropylene and an ethylene-propylene rubber (either EPM or EPDM) although some of the polypropylene may be replaeed by polyethylene. A wide range of blends are possible which may also contain some filler, oil and flame retardant in addition to the polymers. The blends are usually subject to dynamic vulcanisation as described in Section 11.9.1. 

If polypropylene is too hard for the purpose envisaged, then the user should consider, progressively, polyethylene, ethylene-vinyl acetate and plasticised PVC. If more rubberiness is required, then a vulcanising rubber such as natural rubber or SBR or a thermoplastic polyolefin elastomer may be considered. If the material requires to be rubbery and oil and/or heat resistant, vulcanising rubbers such as the polychloroprenes, nitrile rubbers, acrylic rubbers or hydrin rubbers or a thermoplastic elastomer such as a thermoplastic polyester elastomer, thermoplastic polyurethane elastomer or thermoplastic polyamide elastomer may be considered. Where it is important that the elastomer remain rubbery at very low temperatures, then NR, SBR, BR or TPO rubbers may be considered where oil resistance is not a consideration. If, however, oil resistance is important, a polypropylene oxide or hydrin rubber may be preferred. Where a wide temperature service range is paramount, a silicone rubber may be indicated. The selection of rubbery materials has been dealt with by the author elsewhere. ... 

Vulcanised rubbers possess a range of very desirable properties such as resilience, resistance to oils, greases and ozone, flexibility at low temperatures and resistance to many acids and bases. However, they require careful (slow) processing and they consume considerable amounts of energy to facilitate moulding and vulcanisation. These disadvantages led to the development of thermoplastic rubbers (elastomers). These are materials which exhibit the desirable physical characteristics of rubber but with the ease of processing of thermoplastics. 

Carbon blacks are the most widely used fillers for elastomers, especially vulcanised natural rubber. They cause an improvement in stiffness, they increase the tensile strength, and they can also enhance the wear resistance. Other particulate fillers of an inorganic nature, such as metal oxides, carbonates, and silicates, generally do not prove to be nearly so effective as carbon black. This filler, which comes in various grades, is prepared by heat treatment of some sort of organic material, and comes in very small particle sizes, i.e. from 15 to 100 nm. These particles retain some chemical reactivity, and function in part by chemical reaction with the rubber molecules. They thus contribute to the crosslinking of the final material. 

Report 92 Continuous Vulcanisation of Elastomer ProfUes, A. Hill, Meteor Gummiwerke. Report 119 Rubbers in Contact with Food, M.J. Forrest and J.A. Sidwell, Rapra Technology Ltd. 

It is of interest to examine the development of the analytical toolbox for rubber deformulation over the last two decades and the role of emerging technologies (Table 2.9). Bayer technology (1981) for the qualitative and quantitative analysis of rubbers and elastomers consisted of a multitechnique approach comprising extraction (Soxhlet, DIN 53 553), wet chemistry (colour reactions, photometry), electrochemistry (polarography, conductometry), various forms of chromatography (PC, GC, off-line PyGC, TLC), spectroscopy (UV, IR, off-line PylR), and microscopy (OM, SEM, TEM, fluorescence) [10]. Reported applications concerned the identification of plasticisers, fatty acids, stabilisers, antioxidants, vulcanisation accelerators, free/total/bound sulfur, minerals and CB. Monsanto (1983) used direct-probe MS for in situ quantitative analysis of additives and rubber and made use of 31P NMR [69]. 

Deformulation of vulcanised rubbers and rubber compounds at Dunlop (1988) is given in Scheme 2.3. Schnecko and Angerer [72] have reviewed the effectiveness of NMR, MS, TG and DSC for the analysis of rubber and rubber compounds containing curing agents, fillers, accelerators and other additives. PyGC has been widely used for the analysis of elastomers, e.g. in the determination of the vulcanisation mode (peroxide or sulfur) of natural rubbers. 

Phenolic antioxidants in rubber extracts were determined indirectly photometrically after reaction with Fe(III) salts which form a red Fe(II)-dipyridyl compound. The method was applicable to Vulkanox BKF and Vulkanox KB [52]. Similarly, aromatic amines (Vulkanox PBN, 4020, DDA, 4010 NA) were determined photometrically after coupling with Echtrotsalz GG (4-nitrobenzdiazonium fluoroborate). For qualitative analysis of vulcanisation accelerators in extracts of rubbers and elastomers colour reactions with dithio-carbamates (for Vulkacit P, ZP, L, LDA, LDB, WL), thiuram derivatives (for Vulkacit I), zinc 2-mercaptobenzthiazol (for Vulkacit ZM, DM, F, AZ, CZ, MOZ, DZ) and hexamethylene tetramine (for Vulkacit H30), were mentioned as well as PC and TLC analyses (according to DIN 53622) followed by IR identification [52]. 8-Hydroquinoline extraction of interference ions and alizarin-La3+ complexation were utilised for the spectrophotometric determination of fluorine in silica used as an antistatic agent in PE [74], Also Polygard (trisnonylphenylphosphite) in styrene-butadienes has been determined by colorimetric methods [75,76], Most procedures are fairly dated for more detailed descriptions see references [25,42,44],... 

FAB has been used to analyse additives in (un) vulcanised elastomer systems [92,94] and FAB matrices have been developed which permit the direct analysis of mixtures of elastomer additives without chromatographic separation. The T-156 triblend vulcanised elastomer additives poly-TMDQ (AO), CTP (retarder), HPPD (antiozonant), and TMTD, OBTS, MBT and A,lV-diisopropyl-2-benzothiazylsulfenamide (accelerators) were studied in three matrix solutions (glycerol, oleic acid, and NPOE) [94]. The thiuram class of accelerators were least successful. Mixture analysis of complex rubber vulcanisates without chromatographic separation was demonstrated. The differentiation of matrix ions from sample ions was enhanced by use of high-resolution acquisition. 

Ostromow [328] has described the use of conductometry for the analysis of extracts from elastomers and rubbers, such as the determination of various vulcanisation accelerations dithiocarbamates, thiurams (tetramethylthiuramdisulfide, tetramethylthiurammono-sulfide), 2-mercaptobenzothiazole, diphenylguanidine... 

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